Philips Monoknob or frankenstein?

[Dickie]

I collected this from forum member RichardGM a couple of days ago thinking it would be a nice winter project to bring this once-noble set back to its best. On removing the back to have a quick look round I was surprised to be presented with a set of B9A and B7G valves. Looking further the loudspeaker and output transformer were rattling around loose in the cabinet. And then I looked at the back of the rear panel to see a written note “rebuilt in April ’59 “ with all the valve changes noted.

I can only guess that this must be an unfinished project. It seems to have the original mains transformer, but there is no obvious modification to achieve the necessary 6V heater supply. It must have taken some serious work to put in a different set of valves , not exactly equivalents, and change some component values to suit. I wonder if it ever got as far as testing? The loudspeaker was not connected to anything, clearly having its connections severed and replaced with a horrible choc-block. The metal bracket it was resting on is either not original or there is a bit missing, as when all is lined up together the speaker doesn’t line up with it’s hole in the cabinet. Nor is there enough room to fit the output transformer in its normal position. Perhaps the rebuilder lost interest in it before finishing.


So could this have ever worked?

[cathoderay57]

Looks like you have a job of work on your hands, Richard. The picture of the chassis of the 795A at RadioMuseum https://www.vintage-radio.net/forum/...d.php?t=195821 looks like the speaker is original so if you were prepared to refit the original valve holder types (and of course replace all the rotten rubber insulated wire (especially around the mains transformer) restoration looks quite feasible. I guess if it were me, I'd check all the inductors first as the cost of rewinds on top of the valves (unless you already have stock) might make it non cost-effective. I do think these sets perform well and sound excellent though. I have another version, the 785AX, that uses a Pen4DD output valve. Best of luck, Jerry

[Philips210]

Hi.

That's quite an unusual set and I wonder why the person carried out such a significant amount of work to update the valves. Perhaps it was to improve the performance? I think that would seem unlikely though as most Philips sets are usually above average performers.

Personally if it was my set, I'd be looking to restore it back to the original valve line up.

Looks to be a good challenging project.

Regards,
Symon

[stevehertz]

People often changed valves because in their limited knowledge and belief the originals were no longer available. Obsolete was the word. They most probably tried a couple of local radio/TV shops and were faced with a sharp intake of breath through gritted teeth whilst uttering the words, "you won't get those now mate, they're obsolete. The only thing you can do is rebuild it with new valves". That kind of thing. Sad, because as now all know, such valves are relatively easy to get hold of. But of course back then people didn't have the internet or contacts with like minded people as we do now. It appears it was 'easier' to modify the set than go out into the big wild word searching for obsolete valves. In fairness it's not that different now with some solid state devices, especially output devices no longer being available, and people fit replacement ones, modifying the circuit to suit. In another fifty years time someone will be opening up a vintage piece of hifi and saying, "What has been going on here?!"

[Sideband]

I don't see how it could have worked with the original mains transformer....6.3V heaters running from a 4v supply. He hasn't secreted a 6.3V mains transformer somewhere has he?

[cathoderay57]

Looks like you have a nice yellow-label Mullard EL84 in there, suitably sprinkled with magic fairy dust so if it is any good, and you sold that, you could offset the cost of resto by a wee bit. Jerry

[turretslug]

Curious.... I guess everyone has their own take on what's a satisfying way to occupy spare time, but, as has been said, pre-war valves that aren't too scarce now would have been less scarce then. There probably wouldn't have been that much, if any performance improvement- apart from saving a few watts of heater power, and maybe they'd spent too much of their life combatting weird instability with failing metallisation on grid-cap valves. Is there enough gap between mains transformer bobbin and lam stack to wind some more turns to augment the heater winding?

The socket adapter plates look competent enough, perhaps they were an off-the-shelf item back in the day.

[NottsIan]

Hmm, heavily modified over 60 years ago and still survives. I find it hard to believe it has never worked since the "rebuild". Personally, I'd study it very carefully and try to get into the head of the person who rebuilt it and then get it working again. The strange modifications are, after all, part of its history and still a talking point all these years later.

2d

Ian

[turretslug]

They could have connected rectifier and main heater windings in series to make an 8V supply and used an appropriate power resistor to drop to 6.3V for a parallel array including U709. Clunky, but not altogether barmy.

[Dickie]

Thanks to everybody for their comments. I think the first steps will be to make the mains wiring safe and then to power it up without the EZ81 and see how we go.

[cathoderay57]

Originally Posted by turretslug

They could have connected rectifier and main heater windings in series to make an 8V supply and used an appropriate power resistor to drop to 6.3V for a parallel array including U709. Clunky, but not altogether barmy.

This could have been done but not a good solution because the original 1821 rectifier has only a 1A filament and the replacement B9A/B7G valve selection plus the pilot lamps would draw considerably more than that causing an overload on half the series-connected heater supply. Jerry

[PJL]

I have a scrap 797A you can have for the cost of postage. The chassis is a bit rusty but it is basically the same but without the TV4.

[Dickie]

That's a kind offer PJL. I'll drop you a pm.

[turretslug]

I see what you mean, Jerry- I'd been thinking of one of the "traditional" British 2.5A rectifiers but the 1821 is very abstemious by comparison, it's striking that Continental domestic set rectifiers like the 1821 and AZ1 and successor AZ31 use little more heating power than the much later EZ80. To be fair, the British and US 10 watt heater rectifiers had current reserve for hungrier circuits than a straightforward domestic radio. Come to think of it, the U709 seems quite beefy for a set like this when the EZ80 would seem sufficient.

I suppose it comes down to how generous the wire gauge in the rectifier heater winding is- with less overall heater power, the core temperature and I squared R loss in the main heater winding would be lower, so a bit more but not much more could be taken from the rectifier winding. Personally, I would go for a separate 6.3V transformer if there wasn't space for supplementary turns on the main transformer, a transformer of 20-25VA capability would be inexpensive and compact- quite a few years ago, I measured a selection of 6V and 6.3V 20VA transformers from different suppliers and found that there was very little difference in on- and off-load voltage, if anything 6V ones averaged slightly higher- I wondered if the 6V ones were designed catering for solid-state rectifier loss allowance whilst the 6.3V ones were expected to be used in AC situations.

Colin

[RichardGM]

I hadn't looked in here for a few days until alerted by a PM from Dickie. I'm sorry to have lumbered him with a much worse job than expected. I can only apologise on behalf of my late friend, assuming the modifications were his work. It may or may not shed light on his thinking that he was Israeli, had done his national service in the Israeli airforce, had married an Englishwoman (who survives him, now 92 years old) and had eventually settled here and worked in electronics. I don't know all the firms he worked for but for quite a while, possibly up to retirement, he worked for Livingston Hire. I assume that some of the electronic stuff that he collected was throw-outs from Livingston. He had an amateur radio licence. I don't know his call sign off hand but may be able to find out.

[Dickie]

I have managed to spend some time on this. I removed it from the cabinet, gave it a good vacuuming out, replaced all the mains wiring, and fitted a new reservoir/smoothing cap. and the AF output grid coupling cap. Then found the volume control bowden cable was stuck and discovered the volume control was jammed and needed a rebuild (not for the first time, I discovered). Bringing it up slowly on a variac produced some noises but it's very deaf. Radio 5 live is just about there, and oddly if you wave a screwdriver around the middle section of the tuning capacitor it gets much louder. So I guess there is something awry with the alignment.


I'm still in two minds over this. There's a lot of work to get this into a working presentable condition. I didn't mention the loudspeaker has a rust-filled airgap. You might see it back in the "sets offered" section!

Originally Posted by turretslug

They could have connected rectifier and main heater windings in series to make an 8V supply and used an appropriate power resistor to drop to 6.3V for a parallel array including U709. Clunky, but not altogether barmy.

Well, that's almost what they've done. What we all didn't realise was that the main heater winding is centre-tapped. So generating 6V becomes easy. But as has been pointed out, that results in a major overload of the rectifier winding. my brief sums suggest a winding designed for 1A is now loaded with over 2.5A.

[cathoderay57]

Well, it's a big cabinet so you have got plenty of room to install a 6.3v heater transformer. I guess it boils down to whether the set can be persuaded to work acceptably with the more modern valves without a vast amount of effort, or whether a similar amount of work would be needed to revert it to design standard. I'm following this thread with interest; keep us posted! Jerry

[Dickie]

The list of remedial work gets bigger. Today I managed to get MW working briefly with a concerted attack on the wavechange switch with switch cleaner and cotton buds. It is evident that the switch wafers will need a serious clean that can only be done by removing them. Any amount of "coaxing" could not get LW to function.

As you say, it's a (very) big cabinet and every inch on my shelves has to be justified. Initially I thought it would be good to have an example of a "monoknob" set but even if I spent a long time getting this up to a good standard I would not want to keep it. I've therefore decided to offer it as-is on the forum.